Long fasts, especially if you’re never done them before, should definitely be supervised by a physician. To me, “long” is anything longer than a week.
But depending on your body fat percentage, and general state of health? A very fat person could not have a single solid meal for months and not experience any significant health risks, as long as they supplemented. Not suggesting you try that, but it’s a medical fact.
My longest fast has been 32 days. If anything, I was healthier at the end than the beginning! But if I can be graphic, I regret not giving myself an enema when I began. When you don’t eat, your don’t experience peristalsis, the contractions in the intestine that move food through the gut. Pooping a 32-day-old turd was traumatizing, just absolutely rock-hard.